International environmental conventions and trade agreements include provisions requiring national and regional legislation for the establishment of more stringent specifications on motor vehicle fuels to meet stricter air pollution emission standards from automobiles and other motor vehicles. Reduction in sulfur compounds is included in the new fuel specifications.
Engine manufactures are also establishing stricter fuel specifications for their new engines to meet these emission standards. Sulfur in gasoline reduces the effectiveness of catalytic converters and on-board diagnostic equipment in vehicles, thus increasing tailpipe emissions. The reduction of sulfur content in gasoline for internal combustion engines is included as one of the requirements in these more stringent specifications.
The sulfur containing compounds typically found in refinery feedstocks, and which be removed in order to meet impending regulatic thiophene, methylthiophene, tetrahydrothiophene (THT) C2-thiophenes, C4-thiophenes, thiophenol, methylthiophenol, benzothiophene, methylbenzothiophenes and alkylbenzothiophenes. Meeting the 30 ppm specification, which is anticipated for 2005, is projected to cost from 2–5 cents per gallon and will require very substantial capital investments by refiners.
As used herein ultra-low sulfur gasoline means naphtha gasoline products containing no more than 10–30 ppm of sulfur.
As used herein thiophenes and thiophenic compounds refer to all thiophene and alkylthiophenes and alkylthiophenic compounds; and benzothiophenes and benzothiophenic compounds refer to all benzothiophene, alkylbenzothiophenes and alkylbenzothiophenic compounds.
A used herein, the term distillation column reactor means the concurrent reaction and fractionation of a process stream in a column. For the purposes of the present invention, the term catalytic distillation includes reactive distillation and any other process of concurrent reaction and fractional distillation in a column.
The gasoline produced from the effluent of fluid catalytic cracking (FCC) units contributes more that 90% by weight of the sulfur in the gasoline pool. The major portion of the sulfur content in the naphtha stream is attributed to thiophene, thiophenic compounds, benzothiophene, benzothiophenic compounds and other high boiling range +430° F. compounds (alkylbenzothiophenes, benzothiophenes, for example). Unless removed, these heavier compounds can be entrained in the finished gasoline product. A reduction in FCC naphtha sulfur would be a significant step in meeting the current and future specifications for a cleaner burning and more efficient fuel.
Methods for the desulfurization of FCC naphtha are well known in industry. The following four prior art refinery processing options for reducing sulfur content in FCC naphtha are available:
1. Pretreatment of FCC feed—                Pretreatment of the FCC feed, i.e., hydrotreating the feed, can reduce the sulfur in all products produced from the FCC, including the naphtha. However, to reach the low sulfur levels required for the foreseeable “ultra-low sulfur” specifications, post-treatment is still required.        
2. Reducing the naphtha distillation endpoint—                Reducing the naphtha endpoint by undercutting the naphtha product can drop the thiophenic and benzothiophenic compounds from the naphtha into the light cycle oil (“LCO”), thereby reducing the sulfur content of the naphtha. This process reduces the volume of desired gasoline produced and does not reduce sulfur in the light fractions.        
3. Increase sulfur conversion in the FCC reactor—                Increasing sulfur conversion by the use of catalytic changes, by catalyst, reactor additives or by operating changes can only achieve a modest reduction in the sulfur content of the end products. In the finished FCC gasoline, this so-called “trimming” still requires further post-treatment to significantly reduce the sulfur content.        
4. Post-treatment of the FCC naphtha—                Post-treatment of the FCC naphtha utilizing a traditional hydrotreating process is currently the most widely-used method of reducing sulfur to the desired low levels. There are currently numerous commercial and proprietary processes for post-treatment hydrotreating of FCC gasoline. The principal drawback to the use of this process is that post-hydrotreating reduces both the octane and volume of gasoline produced.        
As used herein, the term “full-range naphtha” means the petroleum fraction from C5+ to 430° F. final boiling point. As used herein “gasoline” or “FCC gasoline” refers to naphtha that is to be blended with other components to produce a finished gasoline for use as an automotive fuel.
One post-treatment process utilizes prior art catalytic distillation that is a “non-traditional” hydrotreating process. This prior art catalytic distillation process transfers stabilized full range naphtha from the FCC unit to a naphtha splitter and splits the naphtha by distillation into three cuts, i.e., light cat naphtha (LCN), medium cat naphtha (MCN) and heavy cat naphtha (HCN). The LCN is further processed in a hydrotreating unit. The MCN and HCN are subjected to a hydrodesulfurization process. This process utilizes an outside distillation column that splits the naphtha first and then desulfurizes the naphtha.
The processes known to, and disclosed in the prior art for use in reducing sulfur in gasoline require major capital investments. Accordingly, further substantial capital investments will be required if these prior art processes are utilized to produce gasoline that meets the stricter ultra-low sulfur specifications of governmental environmental regulations and of the engine manufacturers.
It is therefore an object of the present invention to provide an improved process for desulfurizing naphtha/gasoline fractions that retains the high octane naphtha components and does not reduce the volume of naphtha fractions recovered from the FCC unit.
It is a further object of the invention to provide a process that minimizes the capital expenditures required to meet governmental regulations and allow refinery-produced finished gasolines having ultra-low sulfur content to be sold for use in the world markets that have established requirements for this type of fuel.
Yet another object of the invention is to provide an improved desulfurization process that is easy to control and that maximizes the end-product gasoline.